Thursday's vice presidential debate marks the next turn of the U.S. presidential race, as two men who embody the philosophical differences between the political parties more starkly than the candidates at the top of the tickets go toe to toe.

Democrats are counting on Vice President Joe Biden, 69, an old-school party veteran at home in union halls and firehouses, to discredit Republican nominee Mitt Romney’s agenda. Republicans are pushing for Paul Ryan, 42, a self-proclaimed “young gun” of conservatives, to attack President Barack Obama’s economic stewardship and maintain the momentum Romney gained in the first presidential debate on Oct. 3.

Biden and Ryan also will have to use their ties to core party supporters to boost turnout in an election in which polls show as few as 4 percent of likely voters are undecided.

Obama’s lackluster performance in the earlier encounter magnifies the importance of Thursday’s 90-minute debate, starting at 9 p.m., New York time, said Michael Berman, a former senior aide to then-Vice President Walter Mondale.

“I can’t think of a vice presidential debate that’s had this high a level of attention because of something that’s already happened,” said Berman, now president of the Duberstein Group, a Washington lobbying firm founded by Ronald Reagan’s former chief of staff. The stakes are greater even than Biden’s debate with the 2008 Republican vice presidential nominee, Sarah Palin, whose selection excited broad public interest, he said.

Obama was criticized even by Democrats after the first debate for lacking energy and failing to counter Romney’s arguments. The NBC television comedy show “Saturday Night Live” parodied his performance with an actor dozing off at the debate podium.

The president fell from an average 5-point lead over Romney among registered voters during the three days before their first contest to a tie during the three days afterward, according to a Gallup tracking poll. While Romney’s post-debate gains among registered voters have receded, Gallup says he’s now tied with Obama among likely voters, who are more Republican-leaning.

More voters view Biden unfavorably than favorably. Opinion on Ryan is more evenly divided. Thirty-nine percent of registered voters hold a positive opinion of Biden compared with 51 percent who have a negative opinion, according to a poll conducted Oct. 4-7 by the Pew Research Center for the People & the Press. Ryan is viewed favorably by 44 percent and unfavorably by 40 percent. The survey of 1,201 registered voters has a margin of error of plus or minus 3.3 percentage points.

When the vice presidential candidates take the stage at Centre College in Danville, Kentucky, they are likely to deploy the forceful arguments they’ve used on the stump.

Biden campaigns in the scrappy language of the white working-class voters he grew up among in Scranton, Pennsylvania, distilling Obama’s record into a bumper-sticker phrase, “Osama bin Laden is dead and General Motors (GM) is alive.” He has praised the president as a “gutsy” leader.

Ryan has fired off criticism at the Democratic incumbents, telling the Republican National Convention that Obama’s vision for America is “a dull, adventureless journey from one entitlement to the next, a government-planned life, a country where everything is free but us.”

They also offer a clear contrast on approaches to issues such as Medicare, Medicaid and Social Security. Those popular programs, which provide health insurance and pensions to more than 100 million middle- and lower-income Americans, are also key drivers of the nation’s long-term budget deficits.

Ryan sponsored a House Republican budget that would turn the Medicare health-insurance system for the elderly and disabled into a “premium support” plan, in which a fixed amount of money is provided to beneficiaries for the purchase of coverage. Romney also has adopted a premium support plan.

Democrats say the ceiling on Medicare benefits that Republicans support would raise out-of-pocket health-care costs for the elderly.

Romney would turn the Medicaid insurance system for the poor, which also covers costs for nursing-home care for many elderly Americans, into a block-grant program for states. That would mean $1.26 trillion would be stripped from Medicaid over nine years, a Bloomberg Government study released last month found. The program is jointly funded by the federal government and the states.

Presidential candidates, especially in the Republican Party, often have named running mates that strengthen their appeal to the ideological base, among them Dwight Eisenhower’s choice of Richard Nixon in 1952, Gerald Ford’s selection of Robert Dole in 1976 and Dole’s choice of Jack Kemp in 1996.

On the Democratic side, Jimmy Carter’s pick of Mondale in 1976 is an example, said Joel Goldstein, a scholar of the vice presidency and law professor at St. Louis University.

Still, Goldstein said, “I don’t think we’ve ever had a vice presidential candidate who was so identified with a controversial central issue of the campaign as Ryan has been.”

While Nixon was at the forefront of efforts to purge the U.S. government of suspected communists, and 1964 Democratic nominee Hubert Humphrey was an early advocate for civil rights legislation, “their name wasn’t on it in the same way” as Ryan’s plans for the overhaul of Medicare and other entitlement programs, Goldstein said.

Biden has a closer link than Obama to such traditional Democratic constituencies as unions, the elderly, Jewish organizations and women’s groups, Goldstein said.

Biden, the oldest of the four men who will stand at the podium in the debates, also is steeped in his party’s commitment to social programs that support retirees, said Howard Dean, a former Democratic National Committee chairman and onetime presidential candidate.

“Joe has a very deep connection with the old Democratic Party, which is all about the Great Society and the New Deal,” Dean said. “He’s not likely to give up the store in terms of Medicare and Social Security, and people know that.”

Within the Obama administration, Biden also has been a voice for the liberal side on issues such as reducing the U.S. troop commitment in Afghanistan.

Both vice presidential candidates have embraced the presidential running mate’s traditional role in attacking the opposing camp on the campaign trail and in speeches to their respective party national conventions.

Ryan, a Wisconsin lawmaker who is chairman of the House Budget Committee, portrayed Obama and Biden as politicians of the past who have “run out of ideas,” in a nationally televised speech in August accepting his nomination at the Republican convention in Tampa, Florida.

Biden derided the plans Romney and Ryan have proposed for Medicare as “vouchercare” in his own acceptance speech last month at the Democratic convention in Charlotte, North Carolina. He cast Romney’s approach to economic policy as an elevation of balance sheets over human suffering that he disparaged as “the Bain Way,” referring to Bain Capital, the private-equity firm Romney co-founded.

Both candidates have strayed into controversy in making their cases. Independent fact-checkers found as many as seven statements in Ryan’s acceptance speech that were false or distorted. Republicans expressed outrage after Biden told a racially mixed audience in Virginia in August that Romney’s policies toward Wall Street would “put y’all back in chains.”

Steve Schmidt, chief strategist for Republican presidential candidate John McCain’s 2008 campaign, said the impetus will be even greater on Biden to take up the role as attacker because Obama didn’t effectively counter Romney last week.

“The Republican argument can’t go unrebutted in this debate from the Democratic perspective,” said Schmidt, who helped prepare Palin in 2008 and Dick Cheney in 2004 for vice presidential debates. “All the opportunities that they view as missed opportunities to draw blood, to make a rebuttal, they’ll be looking at Joe Biden to do that.”

Cheney confronted a similar situation in his debate eight years ago with Democratic vice presidential nominee John Edwards, which came after an opening encounter in which President George W. Bush fared poorly, Schmidt said.

Cheney’s performance “brought balance back to the race and stopped the hemorrhaging before the president got back on the stage for a second time,” Schmidt said.

Though the main objective for both Biden and Ryan will be to make the case for the presidential candidates, the philosophical and intellectual grounding each has for his views provides an advantage going in to the event, said former House Speaker Newt Gingrich, a candidate in this election’s presidential primaries who participated in debates among the Republican field.